This engine has nostalgic connotations for me from my childhood. It is the same model as my first steam engine. Unfortunately, I never kept the original.
SEL stands for Signaling Equipment Ltd. It seems a strange name for a toy manufacturer. Presumably, it was originally relevant but the catalogue I have from the 1950s (?) lists only toys. Most of the toys were of an educational or scientific nature and included 5 steam engines. These were 4 stationary models, ranging from the "Minor" to the "Major", and the "Steam Launch Unit" (with no boiler). There were several accessories to go with the stationary engines, but no hull nor even a boiler for the marine engine. SEL later traded under the name Merit.
The Major is the largest of the range and is a twin cylinder model. It is a neat, compact design and well made. Probably, their signaling equipment roots showed through in the quality of materials and manufacture. Most of the brass work, including the boiler, cylinders and burner, were painted with an unusual gold finish, which looked even better when new. Unfortunately, this is now peeling on my example and beginning to look a bit tatty. I'm undecided what to do about this: repaint it, polish it all off or leave well alone.
The steam feed from the top of the boiler has an in-line lubricator just before the split to the two typical single action oscillating cylinders. The main dimensions are: base - 6" x 5 1/2", boiler - 4 3/4" x 1 1/2", cylinders - 3/16" x 3/8" stroke, flywheel 1 3/4" diam.
The drawing on the left is taken from a 1950s SEL catalogue.